This invention is directed to a surveying technique for determining whether a monitored individual carrying a monitoring device is tuned to a given signal source such as a television channel or radio station and, in particular, to the transmission of a survey signal combined with a programming signal which are applied at the receiver so as to produce signals in the audible frequency range with a speaker, the survey signal being converted at the receiver to a non-acoustic signal that is transmitted for detection by the monitoring device to thereby identify the signal source to which the individual is tuned.
It is important for a number of reasons to survey an audience to determine to what extent each of its members is tuned at any given time to a particular source of programming (referred to herein as a "signal source") such as a television ("TV") channel or radio station, including the capability to identify even a specific program and/or a specific advertisement. The use herein of the term TV channel or radio station is to be understood as referring to all signal sources. Advertisers are, of course, interested in knowing the number of people exposed to their commercials and to identify their listeners by economic and social categories. Broadcasters use statistics on audience size and type for setting their advertising rates.
Prior art techniques for obtaining such information involve primarily the following approaches. People within the range of the radio station or who receive a television channel (either over the air or by cable) are contacted by phone and interviewed regarding their listening habits. Each person is questioned about the signal sources which that individual listened to during the previous, say, twenty-four hours. However, this technique is suspect because it is subject to recall errors as well as possible bias introduced by the interviewer. For example, if a specific signal source is mentioned to the person being interviewed, the suggestion may elicit a positive response to a question regarding whether that signal source was viewed even when it actually did not occur. Another technique involves keeping diaries by persons agreeing to act as test subjects. Diary entries are to be made manually throughout the day to keep track of what signal sources are being listened to. The diaries are collected periodically and analyzed. However, this approach is prone to inaccuracies because the test subjects may fail to make entries due to forgetfulness or laziness, or wrong entries can be made due to tardiness in attending to this task. Thus, it can be readily seen that the recall-dependent approach first described above is unsatisfactory because people may not accurately remember what they listened to at any particular time and, also, because of the potential problem of suggestive bias. The diary-based approach is likewise unsatisfactory because people may not cooperate and be as meticulous in making timely diary entries as required to obtain the desired record-keeping accuracy.
The above-described techniques all require a significant and time-consuming effort on the part of the test participants to record their TV viewing and/or radio listening habits. Other techniques are known in which the test participants need only play a passive role. For example, it is known to utilize a survey signal transmitted in combination with a programming signal for producing survey signals in the audible range. As disclosed in U.S. Pat. No. 4,718,106, the periodically transmitted survey signal is detected by a receiver and reproduced audibly by a speaker in the form of an audible signal, or code. It is "audible" by virtue of being in what is known as the audible frequency range of human hearing. More specifically, the speaker produces pressure waves in the air that can be detected by a microphone, for example, and with a frequency that is in what is scientifically regarded as the audible range of human hearing. Such pressure waves, or signals, are sometimes referred to herein as acoustic. An acoustic wave is regarded as being audible irrespective of whether it is actually heard by a person, as long as it can be produced by a conventional speaker and detected by a conventional microphone. The audible acoustic signal is detected by a portable device worn by the monitored individual, and data on the incidence of occurrence and/or the time of occurrence are stored and analyzed.
The survey signal can be transmitted at a point in time assigned to it alone, i.e. during a gap in the programming signal. This could be done at predetermined regular intervals, when the program has a natural break in it, or when it ends. Alternatively, the survey signal can be transmitted simultaneously with the programming signal. For example, as disclosed in U.S. Ser. No. 08/003,325 filed Jan. 12, 1993, a notch filter can remove a narrow band of frequencies from the programming signal, and this band is devoted to the survey signal. Corresponding filtering is then carried out by the receiver. This applies to analog signals. However, the programming and survey signals can likewise be digital. The nature of the survey signal can be such as to be heard by the monitored individual as, for example, a multi-note musical tone, or an effort can be made to lower the volume and/or transmit at the extremes of the audible frequency range so as to avoid as much as possible its being heard.
Use of the speaker to reproduce the survey signal is highly useful in many respects. However, the necessity to reproduce the transmitted survey signal audibly for pick-up by a microphone is also a possible drawback of this technique because, if heard by the listener, the sound can tend to be disturbing depending on volume, frequency of occurrence and content. In order to provide meaningful survey results, an interval of, for example, ten minutes can be set between survey signals. For some purposes, such as to take into account frequent switching among channels, for example, an interval of a minute or less may be needed. This can cause a chopping or interruption of, for example, a musical program at an inappropriate point, and some people can become annoyed just by virtue of this code being repeatedly reproduced within their hearing.
A further complicating factor is that a minimal survey signal amplitude is required in order for the portable monitoring device to be able to pick up the survey signal produced by the speaker. However, what this amplitude must be depends on the distance of the monitored individual from the speaker. If there are several TV sets in the house, due to differences in seating arrangements which normally vary from room to room, the distance between the monitored individual and the TV set is not a constant number. It is difficult to set the amplitude even within a given room, if a number of individuals are being monitored in the same household, as is common, because different seats are used by the individuals which can vary significantly in distance from the TV set, for example, depending on room size. Thus, the amplitude of the survey signal adjusted based on this factor to be minimal, yet detectable, for one room or individual would not work for all possible situations and arrangements. Therefore, at least some of the monitored individuals may hear the audible survey signal if, for example, the amplitude is set for distance X while they sit at X/2.
In view of the above, it is preferable to avoid use of a survey signal which might be discerned by the monitored individual. However, government regulations in some countries may require that signals for commercial radios, for example, must be limited to the audible range. In fact, even though speakers which are now available can reproduce frequencies beyond the audible range of a human being, nevertheless the usable transmission frequencies permitted by government regulations are limited to the audible range because of the need for compatibility with older, lower quality speakers. Thus, there exists a conflict between the respective requirements at the transmission end and the receiving end. At the transmission end, there is the need to transmit a survey signal in the audible frequency range, while at the receiving end it is preferable to reproduce the survey signal so as not to disturb the monitored individuals.
Although the technique disclosed in U.S. Pat. No. 4,718,106 is highly useful in terms of carrying out passive monitoring, it has several additional shortcomings. For example, it cannot monitor signal sources that are listened to on a Walkman type of device, which relies on headphones and has no speaker, or with headphones plugged into a radio or TV set which cut-out the speaker because in either case an acoustic signal is not projected far enough to be detected by the device worn by the monitored individual. Also, the technique disclosed in that patent is vulnerable to fraud because false readings can be created in the device if, for example, someone were to set up a bogus signal source emitting a monitoring signal of interest (say for a particular TV program) in a shopping mall. All consumers wearing the monitoring device who happen to be merely shopping in the mall would then register the monitoring signal, and be recorded as viewers, even though they obviously are not.